On March 6th, a paper by S. Geier et. al., of which Professor Zhanwen Han and members of his group are collaborators, was published in the journal Science. They used the Keck II and Pan-STARRS1 telescopes to observe and analyse a Hypervelocity Star (HVS). The star, known as US 708, was found to be moving at a velocity of 1200km/s, far greater than the escape velocity of the Milky Way, which is 420km/s. The uniqueness of this HVS is that it is probably the remnant of a Type Ia supernova explosion.

Conventional theories on the acceleration mechanisms behind HVSs favor a scenario in which a binary system has a dynamical interaction with a supermassive black hole, in such a way that one star is captured by the black hole, while the other is accelerated into an HVS. However, the only black hole that could have induced this degree of acceleration within the Milky Way is at the center of the Galaxy, and the position and velocity of US 708 rule out the possibility that it could have come from that direction.

Subsequent spectroscopy on US 708 found it to be a rapidly rotating helium star, which is consistent with a remnant helium companion star which should be left behind following a single-degenerate Type Ia supernova explosion, according to a previous model by Han’s group. The rapid rotation is a natural result of tidal locking during the progenitor system’s binary phase, and the high velocity is interpreted to be due to the kick from the explosion itself. This explains the existence of US 708, and provides support for the helium star companion single degenerate model.